An Efficient Nonparametric EWMA Wilcoxon Signed-Rank Chart for Monitoring Location

The statistical performance of traditional control charts for monitoring the process shifts is doubtful if the underlying process will not follow a normal distribution. So, in this situation, the use of a nonparametric control charts is considered to be an efficient alternative. In this paper, a nonparametric exponentially weighted moving average (EWMA) control chart is developed based on Wilcoxon signed-rank statistic using ranked set sampling. The average run length and some other associated characteristics were used as the performance evaluation of the proposed chart. A major advantage of the proposed nonparametric EWMA signed-rank chart is the robustness of its in-control run length distribution. Moreover, it has been observed that the proposed version of the EWMA signed-rank chart using ranked set sampling shows better detection ability than some of the competing counterparts including EWMA sign chart, EWMA signed-rank chart, and the usual EWMA control chart using simple random sampling scheme. An illustrative example is also provided for practical consideration.